Process controllers and process monitors are used in a wide variety of industrial, commercial and scientific applications. A process controller, which may include a microprocessor, receives one or more inputs from sensors and transducers that monitor a process. The process controller evaluates the inputs, determines any necessary action based on user defined criteria and outputs one or more signals for controlling the process. Process controllers may include a digital input card for inputting binary signals which represent the state of a switch, a limit sensor or other device having two states. A digital output card may provide binary output signals for controlling lamps, switches and other binary devices which are involved in controlling the process. In other instances, the process controller receives and transmits binary signals through a digital interface without separate input and output cards. A process monitor receives inputs from sensors and transducers but does not provide outputs for controlling the process.
In many cases, it is necessary to input analog signals to the process controller or process monitor in order to monitor continuously variable parameters, such as temperature, fluid level, pressure, pH, conductivity and the like. It may also be necessary to output analog signals for controlling such continuously variable parameters. Some process controllers can be used with optional analog interface cards. Analog interface cards typically handle four or more analog inputs or outputs and may have additional features, such as digital filtering and scaling of the analog signals. Such analog interface cards provide satisfactory performance, but are relatively expensive. The expense of an analog interface card is particularly undesirable when only one or two analog inputs are required and when the additional features of the typical analog interface card are not required.
Other process controllers do not provide analog input and output capability and operate with only binary inputs and outputs. It has not heretofore been possible to input analog signals to such process controllers or to output analog signals from such process controllers.
The input or output analog signal in a process control application may represent a slowly-varying parameter, such as temperature or fluid level, wherein significant changes occur in times on the order of seconds, minutes or longer. In prior art systems, it has been necessary to incur the expense of an analog interface card in order to monitor such slowly-varying parameters. It is desirable to provide an interface circuit for inputting an analog signal to a process controller or a process monitor without incurring the expense of a high performance analog input card. It is also desirable to provide an interface circuit for outputting an analog signal from a process controller without incurring the expense of a high performance analog output card.